mon: make 'osd crush unlink ..' idempotent

Signed-off-by: Sage Weil <sage@inktank.com>
mon: fix crush unit tests for idempotency

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #196 from ceph/wip-mon-crush2

Wip mon crush2

Reviewed-by: Dan Mick <dan.mick@inktank.com>